This invention relates generally to reciprocating gate valves which move longitudinally between open and closed positions and, specifically, to such a gate valve having an adjustable low stress stem connection structure for connecting an operating stem to the gate member.
In recent years, the pipeline industry and the petroleum production industry have constructed pipelines and other fluid flow handling structures having high operating pressures (e.g. 5,000 psi and above) and increased fluid flow velocities. The need has arisen for the construction of gate valves that can withstand these higher operating pressures and increased fluid flow velocities. It has been determined that under these conditions the valves must be able to withstand greater thrust and torque forces and frequently the metal valve parts are under extreme stress. In valves used in the petroleum industry there exists a phenomenon known as "stress corrosion" which causes a breakdown of stressed metal valve parts when the valve is subjected to certain service conditions such as a hydrogen sulfide environment. It is known that the corrosive reaction is accelerated if the valve parts are allowed to remain under stress for extended periods and that ultimate fracture and failure of the valve parts will likely occur. Thus, it is desirable to have a gate valve in which it is possible to minimize the stresses acting on the valve parts.
"Stress corrosion" is not a new phenomenon and stem connection structures presently exist that attempt to minimize such stresses. U.S. Pat. No. 3,463,446 to Natho shows a stem connection between an operating stem and a gate member which permits movement of the gate member relative to the stem in the direction of fluid flow through the valve. However, there is no provision for axial adjustment of the gate member relative to and independent of the operating stem to maintain the proper alignment of the gate member port with the flow passage. Stress is created in the stem connection structure due to misalignment between the gate member port and the flow passages of the valve when the gate member is in the open position. This misalignment can be caused by the stacking of manufacturing tolerances and results in the development of metal stress to a considerable magnitude in the valve parts. Misalignment also creates a backlash effect causing the gate member to "chatter" or bounce when increased fluid flow velocities are present thereby decreasing the service life of the valve.
Further, it is desirable to provide in the closed position of the gate member a metal-to-metal seal between the operating stem and the valve bonnet. Such a metal-to-metal seal inward of the stem packing relative to the flow passage of the valve protects the stem packing from exposure to high fluid pressures in the valve chamber. In Natho, due to the construction of the stem connection structure, such a metal-to-metal seal is not readily feasible. However, U.S. Pat. No. 4,213,480 to Orum et al. shows such a metal-to-metal seal between the operating stem and the valve bonnet, but no means located below the stem is disclosed to permit either longitudinal movement between the gate member and operating stem in a direction parallel to the flow passage through the valve, or axial adjustment of the gate member relative to the axis of the operating stem to maintain the proper alignment of the gate member port with the flow passage through the valve.